Image sensors for an image-input use such as CMOS (Complementary Metal-Oxide-Semiconductor) image sensors or CCD (Charge Coupled Device) image sensors have achieved improvements in performances with an expanding demand for applications in, for instance, digital cameras and cellular phones each equipped with a camera.
The image sensors, described above, need to be further improved in performances, one of which relates to widening a dynamic range.
The conventional image sensor has a dynamic range limited in the order of, for instance, approximately 3 to 4 digits (60 to 80 dB) and does not cover a value of 5 to 6 digits (100 to 120 dB) of the naked eye or a silver-halide film.
Therefore, work has heretofore been expected on research and development of a high-quality image sensor with a dynamic range in the order of 5 to 6 digits (100 to 120 dB) in level equivalent to those of the naked eye or the silver-halide film. The image sensor with such a wide dynamic range has been expected to have applications, in addition to a digital camera or a cellular phone equipped with the digital camera or the like, an image-input camera for a PDA (Personal Digital Assistant), a camera for an advanced traffic management system, a monitoring camera, a camera for an FA (Factory Automation) or a camera for medical use.
As technology of improving characteristics of the image sensor set forth above, for instance, Non-Patent Document 1 discloses that a so-called on-chip noise canceling technology has been developed. In this technology, in order to enhance high sensitivity and a high S/N ratio, the operations are executed to read out noise occurring in a photodiode of each pixel and a signal resulting from relevant noise added with a light signal to take a difference between those components and noise components are removed to allow only a light signal to be extracted.
However, even with such a method, the dynamic range lies at a value less than 80 dB and, therefore, a further widened dynamic range has heretofore been expected.
For example, Patent Document 1 discloses technology in which as shown in FIG. 1, a floating diffusion region having a small capacitance C1 for high sensitivity in low illuminance and a floating diffusion region having a large capacitance C2 for low sensitivity in high illuminance are connected to a photodiode PD to output an output “out 1” for low illuminance and output “out 2” for high illuminance.
Further, Patent Document 2 discloses technology in which as shown in FIG. 2, a capacitance CS of a floating diffusion region FD is arranged to be variable in a range to cover sensitivities for low illuminance and high illuminance for thereby providing a wide dynamic range.
In addition, double-shooting technology has been developed for the shootings in different exposure times for picking up an image in high illuminance with a short exposure time and picking up an image in low illuminance with a long exposure time.
Moreover, Patent Document 3 and Non-Patent Document 2 disclose technologies of widening dynamic range by connecting a transistor switch T between a photodiode PD and a capacitor C, as shown in FIG. 36, and turning on the switch T for a first exposure time allows photoelectric charges to be accumulated in both the photodiode PD and the capacitor C while turning off the switch T for a second exposure time allows, in addition to preceding accumulation of the charges, photoelectric charges to be accumulated. Here, clear demonstration has been made that if light is incident at a rate higher than that causing saturation, an excess of charges is discharged via a reset transistor.
Further, Patent Document 4 discloses that as shown in FIG. 37, a photodiode PD adopts a capacitor C having a larger capacitance than that of the related art capacitance to comply with the shooting in high illuminance.
Furthermore, Non-Patent Document 3 discloses technology in which as shown in FIG. 38, a logarithmic conversion circuit includes MOS transistors in combination for generating an output upon executing logarithmic conversion of a signal delivered from a photodiode PD to comply with the shooting in high illuminance.
However, according to the methods disclosed in the Patent Documents 1, 2, 3 and the Non-Patent Document 2, mentioned above, or the method of performing the double shooting for different exposure times, the shootings for low illuminance and high illuminance need to be performed at different times. Therefore, a difference exists in moving images resulting from the shootings performed in respective illuminance and an issue occurs with a difficulty caused in matching both of the images.
Further, with the methods disclosed in Patent-Document 4 and Non-Patent Document 3 mentioned above, even though a scheme of complying with the shooting in high illuminance can achieve a wide dynamic range, the shooting in low illuminance results in a consequence of low sensitivity with a degraded S/N ratio and the image cannot have improved quality.
As mentioned above, the image sensors such as the CMOS sensors or the like is difficult to achieve a wide dynamic range while keeping high sensitivity with a high S/N ratio.
Moreover, the above is not limited to the image sensor. In the line sensor, including linearly arranged pixels and the optical sensor with no plural pixels, it is difficult achieve a wide dynamic range while keeping high sensitivity with a high S/N ratio.
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2003-134396
Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2000-165754
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-77737
Patent Document 4: Japanese Unexamined Patent Application Publication (JP-A) No. H5-90556 Non-Patent Document 1: S. Inoue et al., IEEE. Workshop on CCDs and Advanced Image Sensors 2001, page 16-19
Non-Patent Document 2: Yoshinori Muramatsu et al., IEEE Journal of Solid-state Circuits, vol. No. 1, January 2003
Non-Patent Document 3: Journal of Image Information Media, 57 (2003)